Are we not the only Earth out there?

Kepler's transit-spotting photometer may have grabbed the headlines throughout 2010 and 2011, but other planet-hunting techniques and teams continue to produce good results. The wobble method, for example, has led to several exciting discoveries. In April 2007, European astronomers used the wobble method to discover, at that point, the most Earth-like planet ever found.

The planet, called Gliese 581c, is 12,000 miles (19, 312 kilometers) in diameter, or not much larger than Earth (8,000-mile diameter). It orbits the same red star as Gliese 581g, but it makes one complete revolution in just 13 Earth-days. This short orbit would make a planet too hot for life, except that Gliese 581's surface temperature is 1/50th that of our sun [source: Than]. As a result, Gliese 581c's surface temperature ranges from an estimated 32 degrees F to 102 degrees F (0 degrees C to 39 degrees C). The research team that discovered it believes it has a developed atmosphere. The planet might not only have water -- it might be entirely covered by oceans.

And in February 2012, an international team of scientists reported the results of their wobble-based research focused on GJ 667C, an M-class dwarf star associated with two other orange dwarfs located about 22 light-years from Earth. The astronomers were actually hoping to learn more about a previously discovered super-Earth (GJ 667Cb) with an orbital period of just 7.2 days, but their observations led to something better -- GJ 667Cc, another super-Earth with an orbital period of 28 days. The new planet, which sits comfortably in the Goldilocks zone of GJ 667C, receives 90 percent of the light that Earth receives [source: Stephens]. Most of this light is in the infrared spectrum, which means the planet likely absorbs a higher percentage of the energy coming to it. The bottom line: GJ 667Cc may absorb the same amount of energy from its star that Earth soaks up from the sun and may, as a result, support liquid water and life as we know it.

At about the same time the GJ 667Cc team was preparing its results, another team from the Space Telescope Science Institute in Baltimore, Md., published results from a massive project known as the PLANET (Probing Lensing Anomalies NETwork) Collaboration. The results, which were based on six years of microlensing observations, may change the face of planet-hunting the same way quantum mechanics changed physics. The study concludes that there are far more Earth-like planets than super-sized gas giants. In fact, according to the study authors, the Milky Way galaxy alone could contain 100 billion planets, 10 billion of which would likely be small, rocky worlds like our inner planets. Approximately 1,500 of these planets could be within 50 light-years of Earth. Now consider that the Milky Way is just one of billions and billions of galaxies flung across the universe. There could be, literally, an infinite number of planets and an almost uncountable number of Earth-like planets.

The question then becomes, Will humans ever stand beneath an alien sunset or walk among black-leafed plants? We certainly hope so.